Ameba
A one-celled animal that changes shape as it moves about. It's propelled
by pseudopodia, temporary "feet" that form from bulges from the cell membrane.
Causes amebic dysentery, an intestinal disease, and other illnesses.

Bacterium
(plural: bacteria)
A free-living organism, with or without a distinct nucleus, that causes
such diseases as strep throat, scarlet fever, typhoid, bubonic plague,
tetanus, and other serious illnesses. Bacteria may form clumps or colonies.
They have no nucleus, and reproduce by dividing. Some are bacteria
older than your great-grandfather.

Fungus
Organisms that contain no chlorophyll, and live as single cells (like
yeasts) or multi-cellular organisms (mushrooms). A least 40 species of
fungi cause disease in humans, including athlete's foot, ringworm, thrush
(particularly in AIDS patients), and histoplasmosis, an infection of the
lungs.

ParasiteA wide range of pathogens cause such diseases as malaria, schistosomiasis
and scabies. These multicellular organisms generally have complex life
cycles and must live part of their lives in another organism. Malaria
parasites spend part of their life cycle in certain mosquitoes.

ProteinThe only infection protein known at this point is the prion.
When its shape is altered, this normal protein can "infect" other proteins,
causing them to change shape as well. Scientists think prions can cause
TSE diseases (Want The Why Files guide
to mad-cow lingo?) in people, sheep, cows, mink, rats, mice, hamsters
and possibly monkeys.

ViroidA small string or circle of RNA (with only about 300 nucleotides)
that infects mainly plants. Unlike viruses, viroids do not encode for
a protein; rather, their structure causes damage and disease.

VirusThe
smallest agent (aside from the prion) able to cause an infection. Viruses
are not cells; they direct cells to make more viruses but seldom contain
the organelles found in bacteria and other cells. Most viruses are short
strings of DNA. "Retroviruses" including herpes and HIV, are composed
of RNA, a similar genetic material). Viruses are between 20 and 250 billionths
of a meter in length.